Factor VIII coagulant antigen (VIII:CAg) is associated with factor VIII, the protein responsible for factor VIII coagulant activity (VIII:C). The proposed research has two principal objectives related to VIII:CAg: (1) improvement of a recently developed technique to analyze Mr variations in VIII:CAg by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE); (2) employment of this and related methods to investigate the structure-function relationship among VIII:CAg, VIII:C, and von Willebrand factor (vWF) in normal and disease states. The analytical procedure involves mixing the 125I-labeled (anti VIII:CAg)-Fab with samples of plasma, separating the mixture by SDS-PAGE, and identifying the Ab-Agn complexes by autoradiography. Our preliminary experiments with this procedure have demonstrated that VIII:CAg: 1) is not covalently linked to vWF; 2) that it has a major Mr form in whole plasma of 2.4 times 10 to the 5th power, based on the mobility of glycoprotein standards; and 3) that it can be modified by thrombin to yield a detectable antigenic species, Mr equals 1.1 times 10 to the 5th power. We have used the SDS-PAGE 125I-Fab method to examine plasma from uremic patients having bleeding abnormalities correctable by cryoprecipitate infusion. These patients have high levels of Mr equals 1.1 times 10 to the 5th power species not present in normal plasma. Using a related technique we found that the VIII:CAg of a CRM plus hemophiliac and abnormal electrophoretic mobility in the presence of CA2 ions. We propose to extend these studies 1) to examine proteolytic and other modifications of VIII:CAg; 2) to identify the active form of VIII:CAg responsible for VIII:C; and 3) to compare variations in VIII:CAg structure and its function, its interaction with other plasma proteins, and its potential as an indicator of disease severity.